The size of individual html files — chapters — that make up an ePub should not exceed 300Kb, according to ePubPreFlight, and this thread. Presumably this is to deal with eReader limitations, which are unspecified.

(I’ve found tell that you shouldn’t make the html/xhtml files in your ePub eBook “too big”, but finding out what “too big” is seems to be hard. Now the Internet knows the maximum size, and you don’t have to know all the magic keywords.)